So that the prior art relevant to the present invention may be more readily understood, the known prior art will be described in two different aspects.
In a first aspect, in which a hot and a cold body are related to one another such that a thermally conductive device acting therebetween is able to coact with respective bodies through the medium of mutually facing contact surfaces, and such that the thermally conductive device can be clamped in between the two bodies.
A second aspect relates to the same situation, although with the difference that the contact surfaces do not face towards one another and the thermally conductive device cannot be clamped between the two bodies.
The first aspect of the known prior art will be described first.
It has long been known to conduct heat from a hot body to a cold body through the medium of some type of thermally conductive device which applies a force against both the hot and the cold body.
When applying the present invention, it is important that the thermally conductive device applies a force between the component and the cooling body that is adapted to the mechanical strength of the circuit board, so that the combined force exerted by a plurality of thermally conductive devices will not result in deformation of the circuit board and to the detriment of the circuit board components or to the actual connections of said mounted components. For instance, a circuit board that includes surface-mounted components is particularly sensitive to mechanical influences wherewith circuit board deformation may result in total or partial release of the solder with which the legs of the surface-mounted components are affixed to the circuit board.
It is also important that the force exerted by the thermally conductive device between the component and the cooling body will not act detrimentally on the cooling body. For instance, when the cooling body is a cassette wall, the force must be adapted to the mechanical strength of said wall, while taking into account the fact that several such devices will act against the same wall and ensuring that the total force exerted by said devices will not result in detrimental deformation of the wall.
It has long been known to allow the heat that radiates from box-enclosed electronic components to be transferred from respective components to the box walls through the medium of the air gap present between the components and the box. This method is useful with components that radiate very small quantities of heat.
It is also known to fabricate a circuit board, on which components are mounted, in mechanical and thermal contact with one of the box walls, so that heat will be conducted from the components to the circuit board and from there to the box. In this case, however, it is necessary that one side of the circuit board is free from components. Neither is it always desirable to conduct heat from the components down into the circuit board.
It is also known to use different types of filling material between the air gap present between components and box. This application requires the use of some type of thermally conductive filling material. Rubber discs or rubber plates are one example of such filling materials. Although rubber discs or plates are effective, they require a very high contact pressure in order to fill the air gap satisfactorily. It is also known to use a very soft rubber-like material that can be brought to desired shapes that are effective in filling irregular air gaps. This material, however, has a much lower thermal conductivity.
Liquid filled plastic pads are another example of such filling materials, although such pads have a limited useful life and efficacy.
Foam material filled with liquid metal is still another example of such filling materials. This solution is highly expensive, however.
Thermally conductive filling materials and double-sided adhesive tapes are marketed by Chomerics, Inc., U.S.A., Thermagon, Inc., U.S.A. and The Bergquist Co., U.S.A., for instance.
It is also previously known to press between electric components a resilient device that will function to conduct heat from respective components to a cooling body. Different examples of this solution are illustrated in publications U.S. Pat. No. 4,674 005, DE-A1-4 324 214 and EP-A1-0 668 715.
Publication EP-A-0 151 068 describes a cooling system which includes, among other things, a heat transfer device that is in contact with the component to be cooled.
The heat transfer device is pressed against the component via a thermo-deformable or thermo-compressible non-rigid bellows-type device such as to transport a coolant. This coolant transports heat from the heat transfer device into a cooling system.
The cooling system also includes a conduit means through which the coolant flows, and a cooling module which functions to cool the coolant.
The non-rigid device may also include a spring which functions to press the heat transfer device against the component.
Other publications that disclose devices that can be considered to form part of the known prior art are publications SE-B-0 433 021 and EP-A-0 541 456.
The known prior art according to the second aspect will now be described.
It is known to mount a cooling body, such as a cooling flange on or in the vicinity of a circuit board on which hot bodies, such as electric and/or electronic components, are mounted. A cooling "flange" may also be comprised of one side of a cassette in which the circuit board with component is mounted.
In these cases, the cooling flange will often be positioned so that its contact surface does not lie immediately above the body to be cooled, and consequently it is not possible to clamp a thermally conductive device between the two bodies in a natural manner so to speak.
It is also known in this second aspect of the prior art to use different types of filling material, although this is difficult to achieve by virtue of the fact that the material cannot be clamped in between the two bodies.
It is also known to fasten between a cooling flange and a component tongue-shaped strips that function to conduct heat from the component to the cooling flange.
These strips may be comprised of a plurality of mutually superimposed foils which are slidably fastened at one body, or at both bodies, to allow a height variation between the two bodies caused by variations in temperature, for instance.
With the intention of enabling the present invention to be understood more thoroughly and with the intention of simplifying the description of the present invention, the following expressions are used in the description and Claims.
Thermal resistivity and thermal conductivity are used synonymously and describe the ability of a certain material to conduct heat.
Thermal resistance and thermal conductance are used synonymously and describe a total absolute value of the ability of a device to conduct heat, and are determined by the thermal resistivity or thermal conductivity of the material included in the device, and the dimensioning of said device.
When a device is comprised of a material that has a certain thermal conductivity and that is not fully determined with respect to its dimensions, this is defined by the expression thermal conductance. This expression can be illustrated, for instance, by the fact that the thermal conductance of a cylinder comprised of a given material and having a diameter of 2 cm will be higher than the thermal conductance of a cylinder of the same material but having a diameter of 1 cm.
The total thermal resistance of the cylinder is not determined by diameter alone, since the length of the cylinder and the thermal resistivity or thermal conductivity of the material must also be known in order to determine this parameter. Nevertheless, it is possible to discuss the thermal conductance of a device, or part of a device, on the basis of a limiting dimension, such as the diameter of a cylinder, without having knowledge of all dimensions or of the thermal resistivity or thermal conductance of the material.